Spectroscopic and ab initio studies on the conformations and vibrational spectra of selected cyclic and bicyclic molecules

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Abstract

The structure, potential energy functions and vibrational spectra of several cyclic
and bicyclic molecules have been investigated using several spectroscopic techniques
and high-level ab initio and density functional theory (DFT) calculations. Laser induced
fluorescence and Raman spectroscopies were used to study the conformation of 2-
indanol in the electronic ground and excited states. These, along with detailed ab initio
calculations, confirmed the existence of four different stable conformations with the one
undergoing an intermolecular hydrogen bonding being the most stable. A theoretical
two-dimensional surface in terms of the ring-puckering and the hydroxyl group internal
rotation vibrations was constructed. This work was extended to obtain preliminary
insights on the conformations and ring-puckering frequencies of 3-cyclopenten-1-ol
using ab initio and DFT calculations.
Infrared and Raman spectra were also utilized to study the structures and
vibrational spectra of -crotonolactone and 2,3-cyclopentenopyridine (pyrindan). Ab initio results showed that -crotonolactone is rigidly planar in the electronic ground state
and has a nearly harmonic ring-puckering potential function. The calculated vibrational
levels were shown to be in very good agreement with the experimental ring-puckering
frequency from vapor-phase Raman observations.
The structures, vibrational spectra, and potential energy functions of several
cyclic molecules were reinvestigated using high-level ab initio computations, and
detailed vibrational analyses based on DFT-B3LYP calculated frequencies were also
carried out. A number of new insights were presented by re-evaluating the available
experimental data for several cyclopentenes, silacyclobutanes and silacyclopentenes. It
was found that the vibrational spectra of some deuterated cyclopentenes possess
extensive coupling between several ring modes and other low-frequency modes.
Reassignments of these spectra have been proposed. Frequencies from DFT-B3LYP
calculations showed very good agreement with the experimental values for
silacyclobutane and its derivatives. The presence of silicon and halogen atoms did not
affect the accuracy of the DFT calculations. In addition, the ring-puckering potential
energy function for silacyclopent-2-ene was studied and alternative assignments of the
far-infrared results were proposed. The new assignments are in good agreement with
computational results. Silacyclopent-2-ene and its -1,1-d2 isotopomer were shown to be
slightly puckered with barriers of less than 50 cm-1.